The present invention relates to a method and system for deriving or employing a mapping relation for determining coordinate positions of a physical effect on a substrate from a plurality of detectors. More particularly, the invention relates to a touchscreen system with a plurality of corner detectors, applying the mapping relation to accurately determine a coordinate position of a touch from the detector outputs, regardless of configuration and possible manufacturing variations.
The functionality of a touchscreen system (typically including a touchscreen and an electronic controller) requires that there exist a relationship between the physical location of a touch, e.g. by a person""s finger, and some coordinate schema. In general, the coordinate system of choice is a two-dimensional Cartesian system with orthogonal horizontal (X) and vertical (Y) axes. The system accuracy is defined as the error between the physical location of the touch and the location reported by the touchscreen/controller. Typically, system accuracy is expressed as a percentage of the touchscreen dimensions.
A touchscreen system may be considered to have two classes of error, (i) those resulting from the design and implementation of the coordinate transformation method (systematic error), and (ii) those resulting from random unit to unit errors within a given class of sensors (manufacturing variance).
Known conductive touchscreen systems have a transparent substrate with a conductive film, e.g., indium tin oxide (ITO) deposited thereon, which is subject to variation in surface conductivity, i.e., xc2x15% or xc2x110%. A particular additional source of errors in systems employing such substrates is the non-linear variation in sensed probe injection current inherent in the configuration of a generally rectangular substrate with electrodes at the corners. This results in a non-uniform current density at various portions of the substrate, especially near the electrodes. Because of the gross non-linearities, it is generally considered undesirable to attempt to perform a piecewise linear compensation, i.e., directly compensate for repositionable electrode position based on a lookup table calibration procedure. Prior methods have therefore sought to include physical linearization structures, such as complex current injection electrodes, in order to reduce the non-uniformity in surface current density, and to linearize the potentials on the substrate. These complex linearizing structures often include complex conductive patterns, diodes or transistors to redistribute or control the redistribution of currents. Still other methods have sought to apply a mathematical algorithm to compensate for the expected distortions due to the rectangular physical configuration.
The coordinate transformation methods employed in prior systems may be categorized into two basic technologies, herein called electromechanical and modeling, each based on a ratiometric approach, whereby there is an assumed mathematical relationship between measured data and a physical location on the surface of the sensor. Typical distortion of the coordinate values in X and Y of an uncompensated rectangular conductive substrate is shown in FIG. 1
Lookup tables provide an addressable storage for correction coefficients, and have been proposed for use in correcting the output of touch position sensors based on a number of technologies. These systems receive an address, i.e., a pair of X and Y values, which corresponds to an uncorrected coordinate, and output data which is used to compensate for an expected error and produce a corrected coordinate, generally in the same coordinate space as the uncorrected coordinate. Proposals for such schemes range from zero order to polynomial corrections. See, U.S. Pat. No. 4,678,869, incorporated herein by reference. In general, the uncorrected coordinate input to the proposed lookup table is initially linearized, i.e., by physical means or by algorithmic means, as discussed below, so that the lookup table operates in a linearized space. Lookup table data values derived from a calibration procedure thus directly correspond to the calibration data coordinate values, and define calibration regions.
Electromechanical Methods
There is a class of systematic error compensating methods comprising electromechanical modifications to the touchscreen system, seeking to approximate an orthogonal grid of electrical potentials from the characteristics shown in FIG. 1. There are four basic methods (summarized below) in this category. The design of such electromechanical methods addresses the systematic error, described above, for a given class of touchscreen. The nature of these methods often results in a significant current drain on the system and the multiplicity of electrodes and/or resistance patterns leads to a high sensor cost. Further, the management of the corrective methods, e.g. excitation switching, sensing plane selection, electrode selection, etc., mandates an interactive control mechanism that adds to the system cost. To correct unacceptable errors occasioned by manufacturing variances within the given class of touchscreens, additional error correcting methods, Such as table lookup, may be employed for each individual touchscreen.
Bus-Bar Methods
This, the most elementary form of correcting the fundamental distortion characteristics is by creating highly conductive bus bars 3 on opposing axes of the substrate 1 (FIG. 2). Excitation is applied to the bus bars 4 and a conductive coversheet 2 provides for the relocatable electrode. Measurement is made as if the touchscreen were a potentiometer, the position of the xe2x80x9cwiperxe2x80x9d being the location of the touch, in that plane of excitation. The excitation is then switched to a second set of bus bars in an orthogonal plane (in some cases located on the cover sheet 2) to define the second coordinate. This technique is exemplified by U.S. Pat. No. 3,622,105. The principal drawback to this technology is its current drain. Further, in those cases where the cover sheet is employed for the second excitation plane, any coversheet damage will result in positional location errors.
Multi-Feed Methods
Multi-feed technology, typified by U.S. Pat. No. 5,438,168, employs active control of multiple electrodes 10 located around the periphery of the resistive substrate 11, as shown in FIG. 3. The operation of these systems are generally functionally equivalent to that of bus-bar technology, in that linear voltage gradients are generated for sampling by a cover sheet relocatable position sensor. Since all electrodes 10 are located on one substrate 11, it is unaffected by cover sheet damage. However, it is a high current drain system, and requires a large number of interconnections. Failure or degradation of any of its switching elements 12 results in system errors.
Resistive Pattern Methods
Many known of corrective methods include use of resistive patterns 21 on, or external to, the touchscreen 20, in such a sequence that the resistive gradient of the touchscreen 20 is approximately the same across its surface, as shown in FIG. 4. U.S. Pat. Nos. 3,798,370, 4,293,734, and 4,661,655 typify this technique. These systems have the high current consumption associated with electromechanical methods, and, because of the complexity of the resistive patterns 21, are prone to errors resulting from manufacturing variances.
Modeling
A second category of coordinate generation technique is based on mathematical functions, chosen because of assumed mathematical relationships for a given class of touchscreens. These methods result in X and Y values that require further adjustments or corrections either because of inadequacies in the assumptions or because of manufacturing variations, or both.
One method, described in U.S. Pat. No. 4,631,355 and Federico et al., xe2x80x9c17.2: Current Distribution Electrographxe2x80x9d SID 86 Digest, p. 307, relies on an a priori assumption concerning the mathematical distribution for points on a touchscreen. Each plane is extracted by ratiometric methods, and the axial xe2x80x9castigmatismxe2x80x9d of each plane, as exemplified in FIG. 1, is then linearized by the use of a second order polynomial equation whose coefficients are empirically derived. U.S. Pat. No. 4,631,355 notes that manufacturing variation errors on the order of 5% are usual, but does not compensate for them, and therefore would need to be corrected for by additional techniques in order to provide an accurate touch position sensing method. Therefore, Federico et al., xe2x80x9c17.2: Current Distribution Electrographxe2x80x9d SID 86 Digest, proposes storing calibration data in a lookup table, for operation separately from the algorithmic compensation system and as a subsequent step to correct the sensor output.
U.S. Pat. No. 4,806,709 is predicated on the assumption of a linear relationship between signals at an electrode located on the conductive surface and the distance between that electrode and the touch location. Using this assumption, the signal from each electrode is employed in an equation that describes the arc of a circle with its origin at the electrode, with a second equation that defines the touch location as the intersection of two or more of such arcs. An implementation of such an approach would have two principal sources of error, (a) non-linearities in the assumed signal/distance relationship, measured data confirming such non-linearities, and/or manufacturing variances which would lead to an error in the calculation of each arc radius, and (b) the classic problem of positional error caused by the difficulty in resolving the angles of intercept as the arcs approach tangency.
The present invention provides a system for providing an accurately determined coordinate position of a physical effect on a medium with a plurality of sensors, each detecting the effect through the medium. The plurality of sensors are mapped to the output coordinate system through a mapping relation, which requires no predetermined relationship of the sensed effects and the coordinate system. In general, the form of the mapping relation is an equation, e.g., a polynomial consisting of various terms, with the coefficients of the mapping equation determined for each example of the integrated sensor, to account for individual manufacturing variations as well as the systematic relationship of the detectors to the coordinate output.
In a preferred embodiment, a touchscreen is provided, having a conductive rectangular substrate with electrodes at each corner of the substrate. An electrical field is induced or effected by proximity of an element, and the electrical field is measured by the plurality of electrodes. Generally, due to the conductive nature of the substrate, a current distribution between the detectors will be measured, the distribution varying with a position of the element with respect to the substrate. Thus, for each position of the element, a unique set of detector outputs will be obtained. A mapping equation is evaluated to map the detector outputs to a desired position coordinate system. Generally, the desired position coordinate system is a Cartesian coordinate system, although other mappings may be provided.
During a manufacturing procedure, each sensor substrate is individually mapped, using a plurality of test points. These test points need not have any particular positions with respect to the substrate, although a relatively large number are preferably provided, dispersed across the surface of the substrate, or at least that portion which is expected to be used. The physical position of each test point is accurately recorded, along with the detector outputs at that test point. A mapping equation is then defined, based on the recorded data, which optimizes an error of the output coordinate positions with respect to the detector outputs. For example, a least mean square curve fitting may be employed to determine a plurality of coefficients of an equation.
In a preferred embodiment, the form of the equation is predetermined, for sensor systems of a given type, meaning that each sensor system of a given type is provided in conjunction with a set of coefficients, which are evaluated with a mapping equation of the same general form. Of course, a predetermined mapping equation is not required for all embodiments, in which case the format of the mapping equation must be specified.
A particular characteristic of the present invention is that, without need for physical or algorithmic prelinearization, the mapping equation is capable of producing accurate coordinate position output from the detector outputs in a single expression. Therefore, the data stored in memory is not in the form of an addressable error lookup table, but rather of the form of data describing a mapping for a set of sensor data coordinates to touch coordinates, without any presumed linear relationship. Preferably, there are at least three detector outputs for mapping to two coordinate axes. Thus, as a characteristic of one embodiment of the invention, the mapping relation has inputs greater in number, and having no one-to-one correspondence to the outputs.
According to a preferred embodiment, a conductive touchscreen is provided which measures the effect of a touch position on a plurality of electrodes to determine a position of the touch. The touch may inject a current, e.g., in a resistive touchscreen, or perturb an electrical field, e.g., a capacitive touchscreen. In most applications, a rectangular substrate having four corner electrodes is provided, although other shapes and electrode arrangements are possible.
In another embodiment, the physical effect is a localized force applied to a stiff, or force transmissive element. The element is suspended by a plurality of force detectors, which may be resistive, piezoelectric, inductive, optical, acoustic, or employ other known sensor types. The outputs of the force transducer detectors are mapped to a coordinate location of the force application. This mapping accounts for flexion of the element, configuration of the element, force distribution at the detector locations, and manufacturing variation in the element and detectors.
In principal, therefore, the medium conducts a physical effect, which is sensed at a plurality of sensing locations. In many instances, there will be a monotonic relation of distance from the location of the effect to each detector and the detector output, although this is not required. However, it is generally required that each set of detector outputs uniquely correspond to a location. Further, it is preferable that there be a continuous first derivative of the detector responses with respect to location of the effect, allowing a continuous mapping function to be employed. The physical effect need not be electrical or force, and may be magnetic, vibrational or acoustic, or another type of effect.
The present invention does not rely on a presumption of ratiometric sensing of effects.
A number of proposed methods rely on uniformity of a conductive media, to detect an amplitude, distribution or delay of a signal, and are thus subject to errors directly resulting from a failure to meet this criteria. Therefore, according to one aspect of the invention, empirically observed data for the media and system incorporating the media is obtained, in order to define an actual mapping relation of the detector outputs and the location of the effect. This data may be processed to various levels. Preferably, an efficient model is employed, with a limited number of stored coefficients of a polynomial curve-fitting equation. The coefficients are preferably derived by a least mean squares fit. The specific terms used in the polynomial equation may be selected based on a sensitivity analysis, preferably with only terms necessary to achieve a given accuracy employed. In general, because the system is a mapping system rather than a linearization followed by calibration system, the stored coefficients do not individually correspond to regions, locations or coordinates of the medium.
One method of limiting the mapping evaluation equation complexity is to define a number of regions of the media, each region being associated with a set of coefficients. In use, the region of the physical effect is estimated, and the set of coefficients corresponding to the estimated region employed to map the detector outputs to the location of the effect. Therefore, while increased coefficient storage is necessary, the complexity of the mapping relation may be reduced and/or the resulting accuracy increased. In general, the estimation of the region will be a simple mapping of boundary regions based on comparisons of detector output data, and therefore there is no need to define an estimated coordinate position of the location of the effect. Typically, four regions are defined for a rectangular substrate medium, each region corresponding to an area around a corner electrode. In the case of the four regions, or quadrants, the region is determined simply by determining the detector with the largest output signal.
In accordance with the present invention, nonlinearities such as the hyperbolic current distribution distortion of a conductive rectangular substrate with corner electrodes, or nonlinearities of substrates having a rectangular or non-rectangular shape with cylindrical, conic, spherical, ellipsoidal or other curvature or non-planar regions may be corrected to map detector outputs to a coordinate location of a touch. Further, in the same mapping process, manufacturing variations such as surface conductivity variations, electrode configuration variations, cover sheet variations, and the like, may also be corrected. Other aspects of the disturbance may also be measured. The mapping relation thus may compensate, in a unified system, for:
(a) The configuration and properties of the medium;
(b) The number, location and characteristics of each of the detectors;
(c) manufacturing variations of the medium and detectors, and other portions of the system.
The present system applies a mapping relation, determined individually for each sensor system, to correct for both nonlinearities and manufacturing variations to provide a high accuracy location coordinate output. Errors due to manufacturing variations such as non-uniform coating thickness, bubbles or scratches in the coating, differences in the connection resistance of the cover sheet or the fixed sensing electrodes, or variations in the characteristics of the interface electronics are included within the mapping relation.
According to the present system, a mapping relation is determined based on a plurality of empirical measurements, which compensate for the overall and actual properties of the sensor system. Further, the generation of the coefficients for the mapping algorithm may performed internally to the controller or on an external system.
Measurement points must generally be spaced less than one half of the spatial Nyquist frequency of significant variations, and these variations must be actually measured. According to one embodiment of the invention, the mapping algorithm may be implemented to compensate for variations which are actually present, without further complexity. Therefore, it is possible to uniquely define the mapping characteristics of an individual sensor system for the required degree of accuracy, and apply an algorithm having the least necessary complexity. For example, where a particular manufacturing variation occurs in one quadrant of a sensor system, a mapping equation applied for that quadrant may have greater complexity than other quadrants. The format of the mapping equation may stored explicitly or implicitly in the stored data.
Because essentially complete mapping may be achieved through application of the algorithm, the present system does not require physical means for controlling the current distributions through the conductive surface, thus allowing a simple substrate configuration with a plurality of corner electrodes, e.g., four corner electrodes of a rectangular panel, to receive electrical signals. The electrical signals, it is noted, may be of constant current, e.g., a DC signal, or of time-varying current waveform having a constant RMS value, e.g., an AC signal. Advantageously, the corner electrodes need not be sequenced or subject to complex time domain analysis; therefore, a simple current source and transconductance amplifiers may be provided. The present system according to the present invention may be used in both resistive and capacitive sensing systems. The present system also allows superposition of different sensing systems, e.g., static and dynamic signals may be simultaneously measured.
Advantageously, the set of mapping relation coefficients are efficiently stored. Further, the scheme of the present system does not assume a ratiometric relationship of the physical effect and the detector outputs, allowing high performance even with non-uniform and non-linear systems.
The computing load associated with typical position determining equations consists of 26 multiply and 20 addition operations to compute both X and Y coordinates, a load well within the capabilities of typical low-cost processors, such as Intel 8051 and derivatives thereof to process within a suitable time-frame. In fact, the system according to the present invention generally has no requirement for any bi-directional interaction between the touchscreen and the remainder of the system, to accomplish the transformation of sensed signals to location coordinates, thus permitting a low-cost embodiment in which the conventional touchscreen controller may be eliminated, the execution of the algorithms being performed by the host computer that also contains the associated application programs. Host processors in systems commonly interfaced with touchscreen sensors, such as Microsoft Windows compatible computers, have sufficient available processing power to evaluate a mapping relation of a touchscreen sensor and execute application programs, without substantial degradation in performance.
The mapping relation information may be stored in a memory device physically associated with the sensor system, or in a separated memory that is used in conjunction with the system. The relatively small number of coefficients necessary allows use of a small memory device, and since the coefficients may be transferred to a local storage of a processor on device initialization, the speed of the memory is not critical. Advantageously, a serial interface EEPROM, physically associated with the interface electronics of a touchscreen with a host processor is employed to store the coefficients. Other memory devices include rotating magnetic media, e.g., floppy disks and the like, and semiconductor memories. While not preferred, it is noted that creation of the mapping equation may be performed subsequent to the manufacturing process, e.g., following device installation on a host system.
As stated above, a preferred method for determining the coefficients for a mapping equation is the well established method of least squares optimization. In this technique, a set of coordinate values for X and Y are given as the desired output from a mapping polynomial equation, which is a function of detector output values. The difference between the value at each point and the value given by the polynomial is squared. This forms a sort of N dimensional bowl shaped surface which has a minimum value at some point in N space. The coefficients of the polynomial are solved in a manner that produces the minimum error for a given data set (an array of detector output values for a set of specific points on the medium with known or determined locations). Solving for the coefficients involves partial differentiation of the squared error term with respect to each coefficient, setting each equation to zero, then solving the resulting N simultaneous equations. While a generic polynomial may be defined which includes one coefficient for each data point, it is preferred to define a simpler equation, having fewer coefficients, and then optimize the coefficients of the simpler equation based on the available data to optimize the error. It is noted that the lowest mean square error is but one optimization technique, and one skilled in the art may optimize differently, if desired.
Where a term of the mapping algorithm equation is found during the design phase of the sensor to have low significance for the entire range of mapping, it may be ignored. Thus, in an embodiment where the sensor system is divided into quadrants, higher order terms may be selectively evaluated or ignored. Thus, where the mapping space is subdivided, terms with low expected significance in any region of the space may be ignored for that region, allowing reduced processing to produce a corrected output while maintaining accuracy
Therefore, one aspect of the present invention provides algorithmic mapping of electrode inputs based on relocatable probe position by means of a mapping formula or set of formulas, derived from an individualized measurement procedure.
In one embodiment, a mapping region defined by the algorithm is not coincident with, and larger than a measurement region, defined by a particular measured point and the arrangement of the other measured points. Preferably, the mapping algorithm according to the present invention does not exceed second or third order in complexity, although fourth or higher order mapping schemes may be provided within the present scope of invention. It is noted that the mapping relation for each coordinate axis need not be of the same form, especially where the substrate is asymmetric.
In addition to its simplicity and low manufacturing costs, the power requirements for this touchscreen system are minimal, some three orders of magnitude less than conventional resistive touchscreens, thus facilitating its application in battery powered systems.
As stated above, the system according to the present invention is not limited to electrical sensing methods.
It is therefore an object of the invention to provide a method for deriving a mapping equation for determining coordinate positions from a plurality of input values, the input values corresponding to signals sensed by a plurality of condition detectors, associated with a medium having a surface, which conducts signals associated with the condition, the signals varying in relationship with a coordinate position of a condition-effecting element with respect to the surface, comprising the steps of providing measured input values produced at a plurality of determined positions of the condition-effecting element; and processing the measured input values in conjunction with the associated determined positions to produce a set of coefficients of a mapping equation comprising a plurality of terms, each term being a coefficient or a mathematical function of at least one coefficient and at least one input value, the mapping equation relating the input values with a coordinate of a position of the condition-effecting element.
It is also an object of the invention to provide a method for mapping a plurality of detector outputs to coordinate positions, comprising the steps of providing a medium for conducting a physical effect, having at least three detectors for detecting a conducted portion of the physical effect at different positions on the medium; measuring, with the at least three detectors, portions of the physical effect conducted through the medium from an origin of the physical effect; and mapping the measured physical effects from the at least three detectors to a coordinate position of the origin of the physical effect, employing a mapping equation derived for the medium and detectors from empirical data, to account for an actual configuration of the medium and detectors.
A still further object of the invention is to provide a method for deriving a mapping relation for determining coordinate positions with respect to a medium having a surface, from a plurality of input values, the input values corresponding to signals sensed by a plurality of condition detectors, each being associated with the medium, the medium being conductive for signals associated with the condition, the signals varying in relationship with a coordinate position of a condition-effecting element with respect to the surface, comprising the steps of providing measured input values produced at a plurality of determined positions of the condition-effecting element; and processing the measured input values in conjunction with the associated determined positions to derive a mapping relation for relating the input values with a coordinate of a position of the condition-effecting element, said mapping relation operating to directly map the input values to coordinate positions substantially without an intermediate representation of an uncorrected coordinate position.
It is a still further object of the invention to provide a position determining system, comprising a medium, having a surface, transmitting physical effects from one portion to another portion; a plurality of spaced detectors for sensing transmitted physical effects in said medium and each producing a detector output; and a memory for storing a plurality of values of information, corresponding to a mapping relationship of said detector outputs at a plurality of determined positions, with respect to said surface, of a physical effect applied to said medium.
It is another object according to the present invention to provide an apparatus for mapping a plurality of detector outputs to coordinate positions, comprising a medium, conducting a physical effect; at least three detectors, at different positions on said medium, each detecting a conducted portion of said physical effect; and a memory for storing information relating to a mapping of a localized physical effect detected at said at least three detectors to a coordinate position of said location of the physical effect, said stored information including information derived for said medium and detectors from empirical observation, to account for an actual configuration of said medium and detectors.
It is an additional object according to the present invention to provide a position determining system, comprising a medium, having a surface, transmitting physical effects from one portion to another portion; a plurality of spaced detectors for sensing transmitted physical effects in said medium and each producing a detector output; and a memory for storing a plurality of values of information, corresponding to a mapping relationship of said detector outputs at a plurality of determined positions, with respect to said surface, of a physical effect applied to said medium, said mapping relationship being selected from the group consisting of:
(a) a mapping equation comprising a plurality of terms, each term being a coefficient or a mathematical function of at least one coefficient and a value associated with at least one detector output, the mapping equation relating the detector outputs with a position of the applied physical effect;
(b) a mapping function operating to directly map the detector outputs to corrected coordinate positions of physical effects substantially without an intermediate representation of an uncorrected coordinate position; and
(c) a mapping function operating to map a localized physical effect detected by at least three detectors to a coordinate position of said location of the physical effect, said stored information including information derived for said medium and detectors from empirical observation, to account for an actual configuration of said medium and detectors.
These and other objects will become apparent. For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.